In the combustion of a fuel, such as coal, oil, peat, waste, etc., in a combustion plant, such as a power plant, a hot process gas is generated, often referred to as a flue gas, containing, among other components, carbon dioxide, CO2. The negative environmental effects of releasing carbon dioxide to the atmosphere have been widely recognized, and have resulted in the development of systems and processes adapted for removing carbon dioxide from the hot process gas generated in the combustion of the above mentioned fuels.
One system and process previously disclosed is a single-stage Chilled Ammonia based system and method for removal of carbon dioxide (CO2) from a post-combustion flue gas stream. Such a system and process has been proposed and taught in published US Patent Application 2008/0072762 entitled Ultra Cleaning of Combustion Gas Including the Removal of CO2.
In a chilled ammonia based system/method for CO2 removal, an absorber vessel is provided in which an ionic solution is contacted in counter current flow with a flue gas stream containing CO2. The ionic solution may be composed of, for example, water and ammonium ions, bicarbonate ions, carbonate ions, and/or carbamate ions.
The absorber vessel is configured to receive a flue gas stream (FG) originating from, for example, the combustion chamber of a fossil fuel fired boiler. It is also configured to receive a CO2 lean ionic solution supply from a regeneration system. The lean ionic solution is introduced into the vessel via a liquid distribution system while the flue gas stream FG is also received by the absorber vessel via a flue gas inlet.
The ionic solution is put into contact with the flue gas stream via a gas-liquid contacting device (hereinafter, mass transfer device, MTD) used for mass transfer and located in the absorber vessel and within the path that the flue gas stream travels from its entrance via an inlet at a bottom portion of the absorber vessel to its exit at a top portion of the absorber vessel. The MTD may be, for example, one or more commonly known structured or random packing materials, or a combination thereof.
The ionic solution is introduced at the top of the MTD and falls downward through the MTD coming into contact with the flue gas stream FG that is rising upward (opposite the direction of the ionic solution) and through the MTD.
Once contacted with the flue gas stream, the ionic solution acts to absorb CO2 from the flue gas stream, thus making the ionic solution “rich” with CO2 (rich solution). The rich ionic solution continues to flow downward through the mass transfer device and is then collected in the bottom of the absorber vessel. The rich ionic solution is then regenerated via a regenerator system to release the CO2 absorbed by the ionic solution from the flue gas stream. The CO2 released from the ionic solution may then be output to storage or other predetermined uses/purposes. Once the CO2 is released from the ionic solution, the ionic solution is said to be “lean”. The lean ionic solution is then again ready to absorb CO2 from a flue gas stream and may be directed back to the liquid distribution system whereby it is again introduced into the absorber vessel.
Published US Patent Application 2009/0101012 describes a multi-stage CO2 removal system comprising an absorber vessel in which CO2 is removed from flue gas by absorption with an ionic solution, such as an ammonia solution, in different absorption stages operating at different temperatures and different NH3-to-CO2 ratios in the ionic solution. The multi-stage absorber vessel comprises a single sump in which CO2 rich ionic solution is collected for transport to a regenerator.
A potential drawback associated with the NH3 based absorption systems and processes of the prior art is that solids, such as ammonium carbonate and/or bicarbonate, formed by chemical reactions between NH3 and CO2, may cause plugging of the absorption column, resulting in a deterioration of the overall performance of the system.